The present invention relates to the field of wind turbine generators, and more particularly to the field of horizontal-axis wind turbines. One of the principal problems involved in designing horizontal axis wind turbines is wind shear, which is the variation of wind velocity with height above ground level. Wind velocities tend to increase with altitude due to aerodynamic surface drag and the viscosity of air. As a result, turbine blades at the top of the rotation experience higher wind velocities than blades at the bottom of the rotation. If not compensated for in the design of the wind turbine, this vertical wind velocity gradient will subject the wind turbine components to damaging stresses.
In addition to wind shear due to natural differences in wind velocity with altitude, wind shear can also be induced by improper alignment of the main shaft axis, i.e., not facing the axis at the optimal angle with respect to the wind direction. Most often, improper alignment results from changes in wind direction. If there is no wind shear, the rotor axis (the axis around which the blades are rotating) should face directly into the wind so that all blades will experience the same wind speed. If however, the main shaft axis is aligned obliquely to the wind in one direction, blades at the top of the rotation move into the wind, and blades at the bottom of the rotation will move with the wind. This will cause blades at the top of the rotation to experience a greater effective wind speed than blades at the bottom. Conversely, if the orientation of the main shaft is oblique to the wind in the opposite direction, blades at the bottom of the rotation will experience a greater effective wind speed than those at the top. Other sources of wind shear include tower shadow and shadow from neighboring wind turbines in a wind farm. Additionally, in aqueous environments, there are significant differences in the flow rate of the water. Typically water at the top of a flowing stream runs faster than water at the bottom of the stream.
Of vital importance in the design of wind turbine generators is operation of the turbine blades at the optimum tip speed ratio to extract as much power as possible out of the wind. Tip speed ratio is defined as the speed at the tips of the turbine blades divided by the speed of the wind. For example, if the wind is blowing at 20 mph and the blade tips are rotating at 100 mph, then the tip speed ratio is 5. If however, there is a wind velocity difference of 10 mph between the lowest and highest blade positions, the tip speed ratio will vary from 4 to about 7, thereby diverging from the optimum design point with consequent loss of efficiency. Variations in tip speed ratio due to wind shear also cause changes in the angle of attack of the turbine blades, which depends on the speed of the blades relative to the wind speed. The effect is to increase the angle of attack at the top of the blade's path and decrease it at the bottom. In the above example, the angle of attack will be increased by almost 3 degrees at the top and decreased by almost 3 degrees at the bottom. This can result in stall at the top and reduced lift power at the bottom.
The lift generated by turbine blades during rotation is applied both in the direction of rotation and in a backward direction. Forces applied in the direction of rotation (around z axis) are also designated as in-plane forces and forces applied in a backward direction (around x and y axes) are also designated as out-of-plane forces. Because of this, wind shear will cause more backward force to be applied to blades experiencing the greater effective wind speed. The stress produced by this unbalance in backward forces is augmented by the concomitant changes in the angle of attack of the blades. This cyclical stress on the blades and bearings can cause excessive wear, maintenance problems, and shorten the useful life of the wind turbine generator.
The prior art in this field has responded to the problems presented by wind shear through the use of a “teeter pin” that is part of the hub. A teeter pin provides for an additional degree of freedom by enabling the turbine rotor to pivot back-and-forth like a playground seesaw. This back-and-forth rotation results in a balancing of the torque on the blades around the teeter axis because blades experiencing the higher wind velocity move with the wind and blades experiencing the lower wind velocity move into the wind. Such teeter pins are useful as applied to two-bladed wind turbines, as they allow the upper blade to tilt backward while the lower blade tilts forward. Thus, the teetering motion of a two-bladed wind turbine tends to equalize the effective wind speeds for both blades, thereby maintaining a more constant tip speed ratio.
The limited seesaw pivoting enabled by teeter pins is, however, inadequate to compensate for wind shear in turbines having three or more blades. This is because teetering is limited to one blade moving forward and the other moving backward in an equal and opposite manner across a single rotating teeter axis. In view of the aforementioned limitations, there is a need for an improvement to be made to existing technology to combat these issues.